This is extension to https://golang.org/cl/113955 that handled
duplicated "unresolved relocation" errors.

For platforms with trampoline support, additional error is generated
per each undefined symbol. This breaks TestUndefinedRelocErrors test
on these platforms.

Proposed fix:

	1. Changes error text to be identical to normal undefined reloc.
	   If relocation is undefined, jump to it will be unresolved
	   as well.

	2. Introduces a map that can be used by all sites that
	   handle this kind of errors which makes it easier
	   to report such errors exactly once.

Errors on ppc64 before this change (note first 4 lines):

	main.defined1: unresolved inter-package jump to main.undefined()
	main.defined1: unresolved inter-package jump to main.undefined()
	main.defined2: unresolved inter-package jump to main.undefined()
	main.defined2: unresolved inter-package jump to main.undefined()
	main.defined1: relocation target main.undefined not defined
	main.defined2: relocation target main.undefined not defined
	runtime.main_main·f: function main is undeclared in the main package

After this change:

	main.defined1: relocation target main.undefined not defined
	main.defined2: relocation target main.undefined not defined
	runtime.main_main·f: function main is undeclared in the main package

Because of (1), errors output is the same on all platforms now.

Fixes #25753

Change-Id: Ic3084202a6fc5d4a6d2d0a93344f012b37fe58ed
Reviewed-on: https://go-review.googlesource.com/116676
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>